Conventional PLLs typically include a reference oscillator and a reference divider, a phase detector, a feedback divider, a low-pass loop filter, and a voltage-controlled oscillator (VCO). The fundamentals of PLL operation are well understood by contemporary designers.
The low-pass filters used in such conventional PLLs typically have a narrow passband, causing the time constant of the PLL to be large. A large time constant is undesirable for many radio applications of PLLs where a fast lock rate (i.e., the time it takes the PLL to lock onto a frequency) is required. One possible solution to this problem is to use a loop filter with a broader bandwidth. However, that solution has the detriment that it makes the input to the VCO noisier. Thus, lock time would be improved at the expense of performance.
Moreover, in some radio PLL circuits, more than one reference frequency is used, to generate the first local oscillator and the transmit frequencies, and a high-frequency first intermediate frequency is also used. This causes a large difference in operating frequencies. Therefore, the loop bandwidth must be a compromise for both bands of operation. That compromise degrades the lock time performance.
Other radio applications for PLLs, such as frequency scanning systems, require a faster lock time for receive-to-receive transitions than for receive-to-transmit transitions. In such cases, the loop gain must be compromised with respect to the lock times required causing degradation in each lock time.